SOO ON THE STRUCTURE OF GLACIER ICE 



It became clear, therefore, that the markings were neither structure 

 nor stratification, but a pecuhar kind of dirt-marks ; and the next 

 point was to ascertain the conditions of their formation. 



On close examination, the face of the ice-cliff exhibiting these 

 markings appeared to be worn into a sort of wavy or rippled surface, 

 the length of the ripples having a general direction downwards. The 

 close-set veins, on the other hand, traverse the face of the ice, as has 

 been said, nearly horizontally. The whole surface of the ice is more 

 or less dirty, not half-a-dozen square inches being without its little 

 grains of sand and minute gravel, brought down, as I imagine, by the 

 water which continually trickles from above ; but the greater part of 

 this impurity is invisible from a small distance, unless where it is, 

 specially accumulated. 



Such accumulation takes place in two localities ; in the first place,, 

 on the little shelves afforded by the upper and more southerly aspects 

 of the '' ripples " above referred to. Here the dirt accumulates quite 

 independently of the structure, and as a consequence either of the 

 form or of the aspect of the part on which it lodges. 



From a little distance these aggregations appear as spots and 

 patches, but further off they cease to be visible, and the glacier 

 between the horizontal streaks appears white. 



These streaks mark the second locality in which the dirt aggregates. 

 Now whenever I carefully examined the surface of the glacier at these 

 points, I found it to be weathered into large granules, separated by 

 coarse fissures which extended for a considerable depth into the sub- 

 stance of the glacier ; while the parts intermediate between the 

 streaks, and which appear white from a distance, presented very much 

 smaller granules, with fissures proportionately finer, and extending 

 inwards for but a very small distance. In short, where the dark 

 streaks existed, the ice was deeply weathered and coarsely granular, 

 affording lodgment for dirt to a depth of two or three inches ; while 

 the intermediate substance had undergone only superficial weathering, 

 and its finely granular structure afforded but little facility for the 

 intrusion of foreign matters. 



The " dirt-streaks," then, are due to the unequal weathering of the 

 ice ; but why does the ice weather unequally ? On seeking for an 

 answer to this question, I found that every dirt-streak corresponded 

 either with a very large blue vein, or with a closer aggregation than usual 

 of smaller blue veins, while the intermediate substance contained a 

 preponderance of the smallest blue veins ; so that the coarse granules 

 were the result of the weathering of parts of the glacier, composed 

 cither exclusively, or for the most part, of blue ice, while those in 



